Laser gyro is broadly used in modern aeronautical industry and well known to the people in the art. The operational principle and use of the laser gyro will be briefly explained herein. In airborne vehicles such as aircrafts or satellite projectiles use Inertia Navigation System(INS) or Inertia Reference System(IRS) to prevent the vehicles from deviating from the navigation target by precisely measuring the errors in navigation path and posture. A core component of this apparatus is a gyroscope which accurately measures the angular velocity due to a change in the posture of the flying object and, thereby corrects the directional error of the object from the direction set to the navigation target.
Laser gyro measures angular velocity of an object by detecting the phase displacement on the basis of Sagnac Effect, which is caused by the difference in the beam paths of forward-propagating wave and reverse-propagating wave propagating through the closed triangular or rectangular propagation channel around the revolving object. Lock-in phenomena, which accounts for the most significant portion of laser gyro error, can be reduced by minimizing the intensity of the counter-propagating waves generated from the backscattering of the mirrors.
Generally, a conventional laser gyro apparatus which incorporates four reflective mirrors actuates two of the four mirrors to control the width of its lock-in zone. To drive the two mirrors, light detectors are employed to continuously monitor the intensity of the reverse-propagating waves. The amount of mirror actuation is determined in proportion to the intensity of the modulated wave included in the propagating waves, and the mirrors are actuated until one of the following conditions is satisfied.
1. Signal amplitude of one of the two light detectors is minimized. PA0 2. Phase displacement between the signals of the two light detectors becomes .pi.. PA0 3. Signal intensities of the two light detectors are the same.
The two mirrors are actuated to move in such a direction that the circumferential length of the ring resonator remain constant. Thus, if one of the two mirrors moves inwardly toward the center of the ring resonator, then the other one moves outwardly from the center by the same distance. The disadvantage of the prior art described above is that the simultaneous movement of the two mirrors is the only way to control the determining factor of the width of lock-in zone (i.e. location of the light spot on the mirror surface and the distance between the mirrors.) Therefore, a more effective and flexible method and apparatus for minimizing the width of the lock-in zone are required.
To minimize the lock-in phenomena described above, methods for using mirrors of very low scattering characteristics or methods for adjusting the location of gyro mirrors by means of piezo element are widely being used. Apparatus and method for minimizing the effect of lock-in phenomena by offsetting the waves scattered from the mirrors and, thereby minimizing the effect of scattered light are disclosed in U.S. Pat. No. 4,526,469 issued on Jul. 2, 1985. The invention discloses that the waves scattered from mirrors are offset in a gyro having a triangular propagation channel by adjusting the location of the two mirrors by means of piezo elements and adjusting only the phase of the backscattered beam while maintaining the length of closed channel. In the method disclosed, two mirrors aligned with each other are actuated only to move the location of mirror surface without changing the channel length. Thus, control of the location of the two mirrors can be accomplished while maintaining the length of the closed channel constant. On the other hand, in case of the conventional laser gyro having a rectangular channel, lock-in zone is also reduced by adjusting the two mirrors as described above. The present invention, however, discloses a method for minimizing the lock-in phenomena by adjusting three mirrors aligned with each other and increasing the offsetting of the scattered waves thereby.